Nine News reporter Simon Bouda said he and his cameraman came across the woman after hearing a voice calling out and knocking on metal at the ravaged building site in Cambridge Terrace.

He reported that Bodkin was pinned under her desk, about 3m into the rubble, and was only saved from being crushed by the collapsing ceiling by the central leg of her desk.

He said: "We actually heard her calling out and knocking. It was a quiet time and we alerted firefighters that in fact there was a noise coming from inside the building.

"Immediately a dead silence fell over this site and they called out, and sure enough she called out back again."

Bodkin had spent the night trapped in the building after it was destroyed by yesterday's 6.3-magnitude earthquake.

Seventy-five people have been confirmed dead, including one Australian, and another 300 are still missing.

New Zealand Prime Minister John Key has declared a national emergency, telling Christchurch residents the whole of NZ is behind them.

Rescuers are still focusing on the site of the Pyne Gould building with hopes of retrieving more survivors.

Director of New Zealand's Ministry of Civil Defence and Emergency Management, John Hamilton, has said rescue teams have a window of opportunity of only two or three days to recover people still trapped in city buildings.

Hamilton, who is coordinating the enormous rescue and recovery operation, said: "We're reasonably pragmatic and my understanding from international experience that there's a kind of window of opportunity which may only be open for about two or three days to effect a real rescue of people who have been trapped."

"But we are also well aware that there are plenty of stories about how long some people do survive in the buildings.

Fire Commander Mike Hall, the head of New Zealand's fire service, said his officers gave an incorrect report to local news media.

Mr Hall said rescue teams in fact have been withdrawn from the site of the CTV building, which was flattened by the earthquake.

Smoke was pouring out of the office building today and it is believed any survivors would have succumbed to smoke asphyxiation.

Family members waiting outside the site were in tears after being spoken to by police.